The ability to create direct bonds between solid objects has been useful and valuable. There are two basic requirements for achieving useful bonds. First, a sizeable percentage of the atoms on the touching surfaces must be brought into intimate contact with each other and second, the atoms of the touching surfaces must react with each other. Until recent times, the intimate contact required for solid-to-solid reaction bonding could only be achieved when a glass was one of the solids.
In these types of bonds, heat and pressure were used to produce sufficient viscous flow to satisfy the first bonding requirement and undoubtedly the heat energy also promoted the reaction of the atoms of the two surfaces.
The use of an intermediate adhesive layer to bond solid objects together has been a long standing alternative to direct solid-to-solid bonding. The advantage of intermediate adhesives is that these adhesives can have a low viscosity which reduces the requirements on the morphology of the surfaces being bonded or the viscosity of the solid being bonded. Also, the temperature required for the desired surface reaction usually is relatively low. However, adhesives often have a large negative impact on the characteristics of the bonded structure.
In the semiconductor and allied fields where it is desirable and often necessary to maintain the physical and electrical properties of the respective solids at their bonded interface, the use of intermediate adhesives are generally froth with problems and are avoided whenever and wherever possible.
The development of modern semiconductor processing technology has made it possible to produce surfaces smooth enough and clean enough to satisfy the first requirement for solid-to-solid bonding with only elastic deformation of the contacting surfaces. This was first reported by J. B. Lasky et al., 1985 Proceedings IEDM, page 684, for oxidized silicon wafers. This technique permitted the solid-to-solid bonding of semiconductor materials without any of the problems encountered through the use of an adhesive. A key ingredient of this approach is an initial solid-to-solid bonding at room temperature generally called "contacting." This contacting holds the surfaces in intimate contact with each other in a clean high temperature furnace without the complications and potential contamination encountered when externally applied forces are used to hold the surfaces together. It was quickly recognized that this technology also had potential for manufacturing sensors with silicon and quartz structures. However, utilization of the full potential of solid-to-solid bonding to create structures with unique combinations of mechanical, electrical, optical and thermal properties has been slow to develop, and in many practical applications, sufficient bond energy was not obtainable at bonding temperatures that were not detrimental to some part of the total structure.
The invention is an improvement to solid-to-solid reaction bonding by achieving much higher bond strengths at low temperatures which can increase with time to an acceptable value.